An mRNA/ribosome binding and RNA helicase complex called 4F is necessary for translation of eukaryotic mRNA. Eukaryotic initiation factor 4E (4E) is the 25 kDa subunit of 4F that binds to the 5' m7G cap structure of mRNA. The quantity of 4E is a limiting factor in 4F assembly. Interestingly, 4E is over-expressed in a variety of epithelial cell cancers, including breast cancers. Moreover, overexpression of 4E correlates with increased tumor metastases and angiogenesis in vivo. Overexpression of 4E transforms NIH 3T3 cells and increases translational initiation and protein synthesis of several growth stimulatory proteins whose mRNAs are known to have long structured 5' untranslated regions (UTR). This is believed to be due to 4E's ability to overcome translational repression due to extensive secondary structure of the 5' UTR. Down-regulation of 4E levels with antisense RNA in several different cell lines, including the breast cancer cell line MDA-MB-435, significantly diminishes their oncogenic properties and decreases their tumorigenic capacity in nude mice. However, these changes were not associated with altered growth in plastic tissue culture dishes. Cells with down-regulated 4E also contains lower levels of several tumor-associated proteins, including fibroblast growth factor and omithine decarboxylase, known to have highly structured 5' UTRs. A number of site directed mutations in 4E have been created, expressed, and tested in vitro. These mutants have altered binding affinities to mRNA caps and 4G, and should provide useful probes for 4E function in cancer cells. I aim to test the hypothesis that overexpression of 4E induces translational upregulation of a subset of mRNA necessary for tissue invasion and metastasis by analyzing polysome fractions from primary cells overexpressing 4E with gene chips. I will also examine cancer cells with downregulated 4E function.